Method for comprehensive evaluation of shale fracability under the geology-engineering “double-track” system

ABSTRACT

The present invention discloses a method for comprehensive evaluation of shale fracability under the geology-engineering “double-track” system, comprising the following steps: S1: Divide the target horizontal fracturing interval into multiple sampling sections; S2: Establish the reservoir property evaluation factor of each sampling section, and calculate the geological evaluation index of the target horizontal fracturing interval according to the reservoir property evaluation factor of each sampling section; S3: Establish the brittleness factor, natural fracture factor and natural fracture opening factor of each sampling section, and then establish the engineering evaluation index of each sampling section according to these factors; S4: Calculate the engineering evaluation index of the target horizontal fracturing interval according to the engineering evaluation factor of each sampling section; S5: Evaluate the fracability of the target horizontal fracturing interval according to the geological evaluation index and the engineering evaluation index.

CROSS-REFERENCE TO RELATED APPLICATIONS

The application claims priority to Chinese patent application No.202210140128.2, filed on Feb. 16, 2022, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to the technical field of oil and gasfield development, in particular to a method for comprehensiveevaluation of shale fracability under the geology-engineering“double-track” system.

BACKGROUND

As the main method of shale gas development in China, the multi-stageand multi-cluster fracturing of horizontal wells aims to form a fracturenetwork in shale reservoir, which reduces fluid flow resistance andimproves the production efficiency and recovery degree of shale gasreservoir. Before the fracturing of shale reservoir, the shalefracability is evaluated according to the abundance of shale gasreserves, rock brittleness, degree of natural fracture development andother factors, which is favorable for avoiding the inefficient andineffective construction of fracture network fracturing of horizontalwells and reducing the construction loss in the process of reservoirstimulation and has important guiding significance for developing theeconomical and efficient fracturing construction scheme.

Currently, there are many methods for the evaluation of shalefracability, mainly based on the comprehensive evaluation index ofreservoir. However, the current comprehensive evaluation index offracability ignores the essence of geological and engineering impactfactors: geological factors determine the development potential ofreservoirs, and engineering factors determine the difficulty of formingfracture network. The comprehensive evaluation index obtained from theirsimple superposition cannot accurately reflect which fracturing methodis more suitable, especially in extreme conditions, such as: (1) theformation with high development potential and great difficulty offorming fracture network (according to the comprehensive evaluationindex, the fracability of such formation is moderate, but such reservoirjust needs to increase the fracturing scale and free up the shale gascapacity); (2) the formation with low development potential and littledifficulty of forming fracture network (the fracability obtained underthe same evaluation method is also moderate, but such formation shouldbe abandoned to reduce unnecessary fracturing cost).

SUMMARY

To solve the above problems, the present invention aims to provide amethod for comprehensive evaluation of shale fracability under thegeology-engineering “double-track” system, considering the synergisticeffect of geological and engineering factors, accurately guiding thetechnical and economic decision making of fracturing construction, andrealizing the fine fracability evaluation and efficient but low-costdevelopment of shale reservoir.

The technical solution of the present invention is as follows:

A method for comprehensive evaluation of shale fracability under thegeology-engineering “double-track” system, comprising the followingsteps:

S1: Divide the target horizontal fracturing interval into multiplesampling sections;

S2: Establish the reservoir property evaluation factor of each samplingsection, and calculate the geological evaluation index of the targethorizontal fracturing interval according to the reservoir propertyevaluation factor of each sampling section;

S3: Establish the brittleness factor, natural fracture factor andnatural fracture opening factor of each sampling section, and thenestablish the engineering evaluation index of each sampling sectionaccording to the brittleness factor, natural fracture factor and naturalfracture opening factor of each sampling section;

S4: Calculate the engineering evaluation index of the target horizontalfracturing interval according to the engineering evaluation factor ofeach sampling section;

S5: Evaluate the fracability of the target horizontal fracturinginterval according to the geological evaluation index and theengineering evaluation index.

Preferably, in the S2, the reservoir property evaluation factor iscalculated according to the following equations:e _(i) =a ₁ϕ_(i) ′+b ₁ω_(i)′  (1)a ₁ +b ₁=1  (2)

$\begin{matrix}{\phi_{i}^{\prime} = \frac{\left( {\phi_{ie} - \phi_{\min}} \right)}{\left( {\phi_{\max} - \phi_{\min}} \right)}} & (3)\end{matrix}$

$\begin{matrix}{\omega_{i}^{\prime} = \frac{\left( {\omega_{i} - \omega_{\min}} \right)}{\left( {\omega_{\max} - \omega_{\min}} \right)}} & (4)\end{matrix}$

Where, e_(i) is the reservoir property evaluation factor of the samplingsection i, dimensionless; a₁ and b₁ are the weight coefficients ofphysical properties, dimensionless; ϕ_(i)′ is the porosity of thesampling section i, dimensionless; ω_(i)′ is the total organic carboncontent of the sampling section i, dimensionless; ϕ_(ie) is theeffective porosity of the sampling section i, %; ϕ_(max) and ϕ_(min) arerespectively the maximum effective porosity and the minimum effectiveporosity in all sampling sections, %; ω_(i) is the total organic carboncontent of the sampling section i, %; ω_(max) and ω_(min) arerespectively the maximum total organic carbon content and the minimumtotal organic carbon content in all sampling sections, %.

Preferably, when the effective porosity of the target sampling sectionis greater than or equal to 4.5%, a₁ and b₁ are 0.65 and 0.35respectively; when the effective porosity of the target sampling sectionis less than 4.5%, a₁ and b₁ are 0.55 and 0.45 respectively.

Preferably, in the S2, the geological evaluation index is calculatedaccording to the following equation:

$\begin{matrix}{A = \frac{\left( {\overset{\_}{e} - e_{\min}} \right)}{\left( {e_{\max} - e_{\min}} \right)}} & (5)\end{matrix}$

Where, A is the geological evaluation index of horizontal fracturinginterval, dimensionless; ē is the average reservoir property evaluationfactor for all sampling sections, dimensionless; e_(max) and e_(min) arerespectively the maximum and minimum reservoir property evaluationfactors in all sampling sections, dimensionless.

Preferably, in the S3, the brittleness factor is calculated according tothe following equations:

$\begin{matrix}{C_{i} = {{\lambda\left\lbrack {1 - {\exp\left( \frac{M_{i}}{E_{i}} \right)}} \right\rbrack} + \frac{\eta\left( {\sigma_{pi} - \sigma_{ci}} \right)}{\sigma_{pi}}}} & (6)\end{matrix}$λ+η=1  (7)

Where, C_(i) is the brittleness factor of the sampling section i basedon the stress-strain curve, dimensionless; λ and η are the standardizedcoefficients, dimensionless; M_(i) is the softening modulus of thesampling section i, in GPa; E_(i) is the elasticity modulus of thesampling section i, in GPa; σ_(pi) is the peak strength obtained fromthe triaxial compression test at the sampling section i, in MPa; σ_(ci)is the participating strength obtained from the triaxial compressiontest at the sampling section i, in MPa;

The natural fracture factor is calculated according to the followingequations:

$\begin{matrix}{P_{Fi} = \frac{P_{fi} - P_{f\min}}{P_{f\max} - P_{f\min}}} & (8)\end{matrix}$

$\begin{matrix}{P_{fi} = \frac{2E_{i}^{2}}{\left( {K_{1i}^{2} + K_{2i}^{2}} \right)\upsilon_{i}}} & (9)\end{matrix}$

$\begin{matrix}{K_{1i} = {{0.3172\rho_{i}} + \frac{0.0457}{V_{ci}} + {0.2131{\ln\left( {DT}_{i} \right)} \times 0.5041}}} & (10)\end{matrix}$

$\begin{matrix}{K_{2i} = {{2.1332\rho_{i}} + \frac{0.0768}{V_{ci}} + {1.1886{\ln\left( {DT}_{i} \right)} \times 9.1808}}} & (11)\end{matrix}$

Where, P_(Fi) is the natural fracture factor of the sampling section i,dimensionless; P_(fi) is the representation number of natural fracturedevelopment degree in sampling section i, ×10⁶ m⁻¹; P_(fmax) andP_(fmin) are respectively the maximum and minimum representation numbersof natural fracture development degree in all sampling sections, ×10⁶m⁻¹; K_(1i) and K_(2i) are respectively the Type I and Type II fracturetoughness of the sampling section i, in MPa·m^(0.5); ν_(i) is theaverage static Poisson's ratio of the sampling section i, dimensionless;ρ_(i) is the average shale density of the sampling section i, in g/cm³;V_(ci) is the average mud content of the sampling section i, %; DT_(i)is the average acoustic time difference of the sampling section i, inμs/m;

The natural fracture opening factor is calculated according to thefollowing equations:

$\begin{matrix}{P_{Ti} = \frac{P_{t\max} - P_{ti}}{P_{t\max} - P_{t\min}}} & (12)\end{matrix}$P _(ti)=σ_(xi) l _(1i) ²+σ_(yi) l _(2i) ²+σ_(zi) l _(3i) ²+2τ_(xyi) l_(1i) l _(2i)+2τ_(yzi) l _(2i) l _(3i)+2τ_(zxi) l _(1i) l _(3i)  (13)

$\begin{matrix}{l_{li} = \sqrt{\frac{1 - l_{3i}^{2}}{1 + {\tan^{2}\theta_{i}}}}} & (14)\end{matrix}$l _(2i) =l _(1i) tan θ_(i)  (15)l _(3i)=cos α_(i)  (16)

Where, P_(Ti) is the natural fracture opening factor of the samplingsection i, dimensionless; P_(ti) is the fluid pressure when the naturalfracture of the sampling section i is opened, in MPa; P_(tmax) andP_(tmin) are respectively the maximum and minimum fluid pressures in allsampling sections to meet the opening of natural fracture, in MPa;σ_(xi), σ_(yi) and σ_(zi) are respectively the normal stress, tangentialnormal stress and vertical stress of the shaft in the sampling sectioni, in MPa; l_(1i), l_(2i) and l_(3i) are respectively the cosine valuesof the included angle between the natural fracture and the maximumhorizontal principal stress, minimum horizontal principal stress andvertical stress in the sampling section i, dimensionless; τ_(xyi),τ_(yzi) and τ_(xzi) are respectively the shear stress components of thesampling section i, in MPa; θ_(i) is the included angle between thenatural fracture and the direction of maximum horizontal principalstress in the sampling section i, °; α_(i) is the dip angle of naturalfracture in the sampling section i, °.

Preferably, in the S3, the engineering evaluation factor is calculatedaccording to the following equation:

$\begin{matrix}{\frac{3}{f_{i}} = {\frac{1}{C_{i}} + \frac{1}{P_{Fi}} + \frac{1}{P_{Ti}}}} & (17)\end{matrix}$

Where, f_(i) is the engineering evaluation factor of the samplingsection i, dimensionless; C_(i) is the brittleness factor of thesampling section i based on the stress-strain curve, dimensionless;P_(Fi) is the natural fracture factor of the sampling section i,dimensionless; P_(Ti) is the natural fracture opening factor of thesampling section i, dimensionless.

Preferably, in the S4, the engineering evaluation index is calculatedaccording to the following equation:

$\begin{matrix}{B = \frac{\left( {\overset{\_}{f} - f_{\min}} \right)}{\left( {f_{\max} - f_{\min}} \right)}} & (18)\end{matrix}$

Where, B is the engineering evaluation index of horizontal fracturinginterval, dimensionless; f is the average engineering evaluation factorfor all sampling sections, dimensionless; f_(max) and f_(min) are themaximum and minimum engineering evaluation factors for all samplingsections, dimensionless.

Preferably, in the S5, the fracability of the target horizontalfracturing interval is evaluated according to the geological evaluationindex and the engineering evaluation index, specifically as follows:

When the geological evaluation index is within the range of [0,0.1] andthe engineering evaluation index is within the range of [0,1.0], thetarget horizontal fracturing interval is not fracturable;

When the geological evaluation index is within the range of [0.1,1.0)and the engineering evaluation index is within the range of [0,1.0], thetarget horizontal fracturing interval is fracturable.

Preferably, the present invention further comprises the S6: Establishthe fracturing construction scheme for the target horizontal fracturinginterval according to the geological evaluation index and theengineering evaluation index.

Preferably, in Step S6, according to the geological evaluation index andengineering evaluation index, the fracturing construction scheme for thetarget horizontal fracturing interval is established as follows:

When the geological evaluation index is within the range of [0,0.1] andthe engineering evaluation index is within the range of [0,1.0], thefracturing construction of the target horizontal fracturing intervalwill be abandoned;

When the geological evaluation index is within the range of [0.1,0.4]and the engineering evaluation index is within the range of [0,0.7], andwhen the geological evaluation index is within the range of [0.4,0.7]and the engineering evaluation index is within the range of (0.7,1.0],the slickwater fracturing+fiber temporary plugging and diverting will becarried out for the target horizontal fracturing interval;

When the geological evaluation index is within the range of [0.1,0.4],and the engineering evaluation index is within the range of [0.7,1.0],the slickwater fracturing will be carried out for the target horizontalfracturing interval;

When the geological evaluation index is within the range of [0.4,0.7]and the engineering evaluation index is within the range of [0,0.3], andwhen the geological evaluation index is within the range of [0.7,1.0]and the engineering evaluation index is within the range of [0.3,0.7],the large-scale fracturing+fiber temporary plugging and diverting+mediummulti-clustered volume fracturing will be carried out for the targethorizontal fracturing interval;

When the geological evaluation index is within the range of [0.4,0.7]and the engineering evaluation index is within the range of [0.3,0.7],and when the geological evaluation index is within the range of[0.7,1.0] and the engineering evaluation index is within the range of[0.7,1.0], large-scale fracturing+fiber temporary plugging and divertingwill be carried out for the target horizontal fracturing interval;

When the geological evaluation index is within the range of [0.7,1.0]and the engineering evaluation index is within the range of [0,0.3],large-scale fracturing+fiber temporary plugging and diverting+highlymulti-clustered volume fracturing will be carried out for the targethorizontal fracturing interval.

The beneficial effects of the present invention are as follows:

The present invention considers geological factors and engineeringfactors in parallel, abandons their previous simple superposition, formsan orthogonal construction proposal, and maximizes the productionbenefit of single well on the premise of avoiding unnecessaryconstruction waste. It overcomes the defects and deficiencies of theprior art, and it is beneficial to realize the development of shalereservoir with lower cost but higher efficiency, with broad marketapplication prospect.

BRIEF DESCRIPTION OF DRAWINGS

In order to explain the embodiments of the present invention or thetechnical solutions in the prior art more clearly, the following willmake a brief introduction to the drawings needed in the description ofthe embodiments or the prior art. Obviously, the drawings in thefollowing description are merely some embodiments of the presentinvention. For those of ordinary skill in the field, other drawings canbe obtained based on the structures shown in these drawings without anycreative effort.

FIG. 1 is the process diagram of dynamic adjustment and optimization offracturing construction scheme in the present invention;

FIG. 2 is the production curve diagram of Well X in the presentinvention after fracturing in an embodiment;

FIG. 3 is the production curve diagram of Well Y in the contrastiveexample after fracturing in an embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present invention is further described with reference to thedrawings and embodiments. It should be noted that the embodiments inthis application and the technical features in the embodiments can becombined with each other without conflict. It is to be noted that,unless otherwise specified, all technical and scientific terms hereinhave the same meaning as commonly understood by those of ordinary skillin the field to which this application belongs. “Include” or “comprise”and other similar words used in the present disclosure mean that thecomponents or objects before the word cover the components or objectslisted after the word and its equivalents, but do not exclude othercomponents or objects.

As shown in FIG. 1 , the present invention provides a method forcomprehensive evaluation of shale fracability under thegeology-engineering “double-track” system, comprising the followingsteps:

S1: Divide the target horizontal fracturing interval into multiplesampling sections.

It should be noted that the target horizontal fracturing interval isdivided into multiple sampling sections to mainly reduce the impact ofreservoir heterogeneity on the accuracy of evaluation results. If thepresent invention is used to evaluate the fracability of homogeneousreservoirs, the target horizontal fracturing interval may not be dividedinto sections. In addition, when the horizontal fracturing interval isdivided, the number of sections can be calculated according to thelength of the horizontal fracturing interval, and all sampling sectionscan be numbered from downhole to wellhead. It should be noted that thedivision by length is only a preferred solution. The target horizontalfracturing interval can be also divided into several sections in thepresent invention according to other standards, such as the geologicalparameters of the horizontal fracturing interval.

S2: Establish the reservoir property evaluation factor of each samplingsection, and calculate the geological evaluation index of the targethorizontal fracturing interval according to the reservoir propertyevaluation factor of each sampling section.

In a specific embodiment, the reservoir property evaluation factor iscalculated according to the following equations:e _(i) =a ₁ϕ_(i) ′+b ₁ω_(i)′  (1)a ₁ +b ₁=1  (2)

$\begin{matrix}{\phi_{i}^{\prime} = \frac{\left( {\phi_{ie} - \phi_{\min}} \right)}{\left( {\phi_{\max} - \phi_{\min}} \right)}} & (3)\end{matrix}$

$\begin{matrix}{\omega_{i}^{\prime} = \frac{\left( {\omega_{i} - \omega_{\min}} \right)}{\left( {\omega_{\max} - \omega_{\min}} \right)}} & (4)\end{matrix}$

Where, e_(i) is the reservoir property evaluation factor of the samplingsection i, dimensionless; a₁ and b₁ are the weight coefficients ofphysical properties, dimensionless; ϕ_(i)′ is the porosity of thesampling section i, dimensionless; ω_(i)′ is the total organic carboncontent of the sampling section i, dimensionless; ϕ_(ie) is theeffective porosity of the sampling section i, %; ϕ_(max) and ϕ_(min) arerespectively the maximum effective porosity and the minimum effectiveporosity in all sampling sections, %; ω_(i) is the total organic carboncontent of the sampling section i, %; ω_(max) and ω_(min) arerespectively the maximum total organic carbon content and the minimumtotal organic carbon content in all sampling sections, %.

Preferably, when the effective porosity of the target sampling sectionis greater than or equal to 4.5%, a₁ and b₁ are 0.65 and 0.35respectively. When the effective porosity of the target sampling sectionis less than 4.5%, a₁ and b₁ are 0.55 and 0.45 respectively. It shouldbe noted that the physical property weight coefficient of the presentembodiment is the preferred physical property weight coefficient of thepresent invention according to the accuracy of the results, etc. Inaddition to the physical property weight coefficient of the presentembodiment, other physical property weight coefficients can be alsoadopted according to the accuracy and other requirements.

In a specific embodiment, the geological evaluation index is calculatedaccording to the following equation:

$\begin{matrix}{A = \frac{\left( {\overset{\_}{e} - e_{\min}} \right)}{\left( {e_{\max} - e_{\min}} \right)}} & (5)\end{matrix}$

Where, A is the geological evaluation index of horizontal fracturinginterval, dimensionless; ē is the average reservoir property evaluationfactor for all sampling sections, dimensionless; e_(max) and e_(min) arerespectively the maximum and minimum reservoir property evaluationfactors in all sampling sections, dimensionless.

S3: Establish the brittleness factor, natural fracture factor andnatural fracture opening factor of each sampling section, and thenestablish the engineering evaluation index of each sampling sectionaccording to the brittleness factor, natural fracture factor and naturalfracture opening factor of each sampling section.

In a specific embodiment, the brittleness factor is calculated accordingto the following equations:

$\begin{matrix}{C_{i} = {{\lambda\left\lbrack {1 - {\exp\left( \frac{M_{i}}{E_{i}} \right)}} \right\rbrack} + \frac{\eta\left( {\sigma_{pi} - \sigma_{ci}} \right)}{\sigma_{pi}}}} & (6)\end{matrix}$λ+η=1  (7)

Where, C_(i) is the brittleness factor of the sampling section i basedon the stress-strain curve, dimensionless; λ and η are the standardizedcoefficients, dimensionless; M_(i) is the softening modulus of thesampling section i, in GPa; E_(i) is the elasticity modulus of thesampling section i, in GPa; σ_(pi) is the peak strength obtained fromthe triaxial compression test at the sampling section i, in MPa; σ_(ci)is the participating strength obtained from the triaxial compressiontest at the sampling section i, in MPa;

The natural fracture factor is calculated according to the followingequations.

$\begin{matrix}{P_{Fi} = \frac{P_{fi} - P_{f{}\min}}{P_{f\max} - P_{f\min}}} & (8)\end{matrix}$

$\begin{matrix}{P_{fi} = \frac{2E_{i}^{2}}{\left( {K_{li}^{2} + K_{2i}^{2}} \right)\upsilon_{i}}} & (9)\end{matrix}$

$\begin{matrix}{K_{li} = {{0.3172\rho_{i}} + \frac{0.0457}{V_{ci}} + {0.2131{\ln\left( {DT}_{i} \right)} \times 0.5041}}} & (10)\end{matrix}$

$\begin{matrix}{K_{2i} = {{2.1332\rho_{i}} + \frac{0.0768}{V_{ci}} + {1.1886{\ln\left( {DT}_{i} \right)} \times 9.1808}}} & (11)\end{matrix}$

Where, P_(Fi) is the natural fracture factor of the sampling section i,dimensionless; P_(fi) is the representation number of natural fracturedevelopment degree in sampling section i, ×10⁶ m⁻¹; P_(fmax) andP_(fmin) are respectively the maximum and minimum representation numbersof natural fracture development degree in all sampling sections, ×10⁶m⁻¹; K_(1i) and K_(2i) are respectively the Type I and Type II fracturetoughness of the sampling section i, in MPa·m^(0.5); ν_(i) is theaverage static Poisson's ratio of the sampling section i, dimensionless;ρ_(i) is the average shale density of the sampling section i, in g/cm³;V_(ci) is the average mud content of the sampling section i, %; DT_(i)is the average acoustic time difference of the sampling section i, inμs/m;

The natural fracture opening factor is calculated according to thefollowing equations:

$\begin{matrix}{P_{Ti} = \frac{P_{t\max} - P_{ti}}{P_{t\max} - P_{t\min}}} & (12)\end{matrix}$P _(ti)=σ_(xi) l _(1i) ²+σ_(yi) l _(2i) ²+σ_(zi) l _(3i) ²+2τ_(xyi) l_(1i) l _(2i)+2τ_(yzi) l _(2i) l _(3i)+2τ_(zxi) l _(1i) l _(3i)  (13)

$\begin{matrix}{l_{li} = \sqrt{\frac{1 - l_{3i}^{2}}{1 + {\tan^{2}\theta_{i}}}}} & (14)\end{matrix}$l _(2i) =l _(1i) tan θ_(i)  (15)l _(3i)=cos α_(i)  (16)

Where, P_(Ti) is the natural fracture opening factor of the samplingsection i, dimensionless; P_(ti) is the fluid pressure when the naturalfracture of the sampling section i is opened, in MPa; P_(tmax) andP_(tmin) are respectively the maximum and minimum fluid pressures in allsampling sections to meet the opening of natural fracture, in MPa;σ_(xi), σ_(yi) and σ_(zi) are respectively the normal stress, tangentialnormal stress and vertical stress of the shaft in the sampling sectioni, in MPa; l_(1i), l_(2i) and l_(3i) are respectively the cosine valuesof the included angle between the natural fracture and the maximumhorizontal principal stress, minimum horizontal principal stress andvertical stress in the sampling section i, dimensionless; τ_(xyi),τ_(yzi) and τ_(xzi) are respectively the shear stress components of thesampling section i, in MPa; θ_(i) is the included angle between thenatural fracture and the direction of maximum horizontal principalstress in the sampling section i, °; α₁ is the dip angle of naturalfracture in the sampling section i, °.

The engineering evaluation factor is calculated according to thefollowing equation:

$\begin{matrix}{\frac{3}{f} = {\frac{1}{C_{i}} + \frac{1}{P_{Fi}} + \frac{1}{P_{Ti}}}} & (17)\end{matrix}$

Where, f_(i) is the engineering evaluation factor of the samplingsection i, dimensionless.

It should be noted that the brittleness factor, natural fracture factorand natural fracture opening factor in Equation (17) of the presentinvention can be calculated by other calculation methods in the priorart in addition to the calculation equations shown in Equations(6)-(16).

S4: Calculate the engineering evaluation index of the target horizontalfracturing interval according to the engineering evaluation factor ofeach sampling section.

In a specific embodiment, the engineering evaluation index is calculatedaccording to the following equation:

$\begin{matrix}{B = \frac{\left( {\overset{\_}{f} - f_{\min}} \right)}{\left( {f_{\max} - f_{\min}} \right)}} & (18)\end{matrix}$

Where, B is the engineering evaluation index of horizontal fracturinginterval, dimensionless; f is the average engineering evaluation factorfor all sampling sections, dimensionless; f_(max) and f_(min) are themaximum and minimum engineering evaluation factors for all samplingsections, dimensionless.

S5: Evaluate the fracability of the target horizontal fracturinginterval according to the geological evaluation index and theengineering evaluation index, specifically as follows:

When the geological evaluation index is within the range of [0,0.1] andthe engineering evaluation index is within the range of [0,1.0], thetarget horizontal fracturing interval is not fracturable.

When the geological evaluation index is within the range of [0.1,1.0)and the engineering evaluation index is within the range of [0,1.0], thetarget horizontal fracturing interval is fracturable.

In a specific embodiment, the method for comprehensive evaluation ofshale fracability under the geology-engineering “double-track” system inthe present invention further comprises the S6: Establish the fracturingconstruction scheme for the target horizontal fracturing intervalaccording to the geological evaluation index and the engineeringevaluation index. The specific fracturing construction scheme is shownin Table 1:

TABLE 1 Fracturing construction scheme for target horizontal fracturinginterval under different geological evaluation indexes and engineeringevaluation indexes Geo- logical evaluation Engineering evaluation indexB index A [0, 0.3] (0.3, 0.7] (0.7, 1.0] [0, 0.1] Abandoned (0.1, 0.4]Slickwater Slickwater Slickwater fracturing + fracturing + fracturingfiber temporary fiber temporary plugging and plugging diverting anddiverting (0.4, 0.7] Large-scale Large-scale Slickwater fracturing +fiber fracturing + fracturing + temporary plugging fiber temporary fiberand diverting + plugging temporary medium multi- and diverting pluggingand clustered volume diverting fracturing (0.7, 1.0] Large-scaleLarge-scale Large-scale fracturing + fiber fracturing + fracturing +temporary plugging fiber fiber and diverting + temporary temporaryhighly multi- plugging plugging and clustered volume and divertingfracturing diverting + medium multi-clustered volume fracturing

In a specific embodiment, in Table 1, the slickwater formula forslickwater fracturing is 0.2% effective drag reducer FJZ-2+0.5% polymeremulsion viscosifier FZN-1+0.25% anti-water lock surfactant FSSJ-8+100KCl, the construction displacement is 16-18 m³/min, and the liquidintensity is 28-30 m³/m. The specific displacement and consumption arecalculated according to the pumping equipment and the constructionlength of the horizontal interval (the calculation method is the priorart and will not be described here). It should be noted that slickwaterfracturing technology is a prior art. In addition to the slickwaterformula used in this embodiment, other slickwater formulas in the priorart can be also used depending on the formation conditions of the targetwell.

The large-scale fracturing means that the liquid intensity forfracturing is 150% of that for slickwater fracturing, and the otherparameters are the same as those for slickwater fracturing. The fibertemporary plugging and diverting means that the fiber is added when theslickwater pumping pressure is stable until the pumping pressure isincreased by 6-10 MPa; the fiber length is 5.00-6.00 mm, the fiberconcentration is 0.5-1.8%, the fiber consumption is calculated accordingto the on-site construction conditions (the calculation method is theprior art, which will not be described here). The cluster spacingcorresponding to the medium multi-clustered volume fracturing and thehighly multi-clustered volume fracturing is 12 m and 10 m, and theperforating density is 4 shots/cluster and 6 shots/cluster respectively.For other fracturing construction schemes, the cluster spacing is 14 m,and the perforating density is 4 shots/cluster. The perforating depth is0.2 m, and the number of clusters is calculated according to the lengthof the fracturing interval (the calculation method is the prior art,which will not be described here).

Two adjacent horizontal wells X and Y in shale gas reservoir in southernSichuan are taken as examples to verify the accuracy of the method forcomprehensive evaluation of shale fracability under thegeology-engineering “double-track” system in the present invention. Thehorizontal interval length of Well X and Well Y are 1,080 m and 1,260 mrespectively. The geological exploration results show that the shale gasreserves in the reservoir where the two horizontal intervals are locatedare high, but the brittleness index is poor. According to the horizontalwell development experience in this area, Well X and Well Y are subjectto stimulation by staged and clustered fracturing in a horizontal wellwith a section length of 60 m, and the number of sections is 18 and 21respectively. The cluster spacing is 12 m, the number of clusters is 5;the perforating density is 4 shots/cluster, and the perforating depth is0.2 m; the slickwater pumping displacement is 16 m³/min, the averageamount of liquid used per section is 1,800 m³, and the total amount is3.24×10⁴ m³ and 3.78×10⁴ m³ respectively.

The present invention is adopted for the fracability evaluation of WellX, specifically as follows:

(1) Divide the horizontal interval from toe end to heel end every 10 mand number it;

(2) According to the core laboratory evaluation experiment data, countthe total organic carbon content, porosity and effective porosity ofeach sampling section, and obtain the reservoir property evaluationfactor of each sampling section based on the Equations (1)-(4);

(3) Obtain the geological evaluation index of each target fracturinginterval of Well X based on the Equation (5) according to the reservoirproperty evaluation factor of each sampling section;

(4) Obtain other basic data of each sampling section by analyzing themineral composition, sorting out the logging data, measuring the coremechanical parameters, establishing the numerical model and through datafitting and regression, etc. Obtain the brittleness factor, naturalfracture factor and natural fracture opening factor of each samplingsection based on the Equations (6)-(16);

(5) Obtain the engineering evaluation index of each target fracturinginterval based on the Equation (17) according to the brittleness factor,the natural fracture factor and the natural fracture opening factor ofeach sampling section;

(6) According to the geological evaluation index obtained from Step (3)and the engineering evaluation index obtained from Step (5), carry outthe dynamic adjustment and optimization of fracturing constructionscheme for Well X in combination with FIG. 1 , with the results shown inTable 2.

It should be noted that, in Step (6), the dynamic adjustment andoptimization method shown in FIG. 1 is taken for the staged fracturingof target fracturing interval so as to improve the final fracturingeffect. In use, it can be also staged directly according to the sectionlength as required, and then according to the geological evaluationindex and engineering evaluation index results of each section, thefracturing construction scheme of each section can be formulated incombination with Table 1.

TABLE 2 Suggestions for fracturing construction scheme of Well XGeological Engineering Section Measured Section evaluation evaluationNo. depth length index index k m m A_(k) B_(k) Fracturing constructionscheme 1  5420- 70 0.605 0.470 Large-scale fracturing + fiber temporaryplugging and 5490 diverting 2  5360- 60 0.415 0.263 Large-scalefracturing + fiber temporary plugging and 5420 diverting + mediummulti-clustered volume fracturing 3  5290- 70 0.217 0.294 Slickwaterfracturing + fiber temporary plugging and 5360 diverting 4  5240- 500.809 0.281 Large-scale fracturing + fiber temporary plugging and 5290diverting + highly multi-clustered volume fracturing 5  5160- 70 0.3910.439 Slickwater fracturing + fiber temporary plugging and 5230diverting 6  5090- 70 0.582 0.596 Large-scale fracturing + fibertemporary plugging and 5160 diverting 7  5030- 60 0.805 0.372Large-scale fracturing + fiber temporary plugging and 5090 diverting +medium multi-clustered volume fracturing 8  4980- 50 0.759 0.225Large-scale fracturing + fiber temporary plugging and 5030 diverting +highly multi-clustered volume fracturing 9  4930- 50 0.795 0.219Large-scale fracturing + fiber temporary plugging and 4980 diverting +highly multi-clustered volume fracturing 10  4860- 60 0.731 0.554Large-scale fracturing + fiber temporary plugging and 4920 diverting +medium multi-clustered volume fracturing 11  4790- 70 0.532 0.387Large-scale fracturing + fiber temporary plugging and 4860 diverting 12 4720- 70 0.388 0.405 Slickwater fracturing + fiber temporary pluggingand 4790 diverting 13  4640- 70 0.583 0.701 Slickwater fracturing +fiber temporary plugging and 4710 diverting 14  4580- 60 0.706 0.644Large-scale fracturing + fiber temporary plugging and 4640 diverting +medium multi-clustered volume fracturing 15  4510- 70 0.643 0.476Large-scale fracturing + fiber temporary plugging and 4580 diverting 16 4460- 50 0.793 0.256 Large-scale fracturing + fiber temporary pluggingand 4510 diverting + highly multi-clustered volume fracturing

In Table 2, the construction amount of slickwater for slickwaterfracturing is 31,200 m³, the pumping displacement is 16 m³/min, and thetotal amount of slickwater is 3.12×10⁴ m³. The fiber concentration forfiber temporary plugging and diverting is 1%, the fiber length is 6 mm,and the fiber dosage is 7 t. The cluster spacing for mediummulti-clustered volume fracturing is 12 m, and the perforating densityis 4 shots/cluster. The cluster spacing for highly multi-clusteredvolume fracturing is 10 m, and the perforating density is 6shots/cluster. The cluster spacing of other sections is 14 m, and theperforating density is 4 shots/cluster. The perforating depth is 0.2 m.

According to the fracturing construction scheme suggested in Table 2,the production curve of Well X after fracturing is obtained, withresults shown in FIG. 2 . Seen from FIG. 2 , the maximum daily gasoutput of Well X is 26.1×10⁴ m³ at the initial stage of construction,and the average daily gas output is 15.1×10⁴ m³ in the first year and11.5×10⁴ m³ in the second year.

For Well Y adjacent to Well X, the original fracturing design scheme(the above fracturing scheme formulated depending on the experience ofhorizontal well development) is adopted for fracturing construction, andthe production curve of Well Y after fracturing is obtained, withresults shown in FIG. 3 . On the one hand, the real-time detection ofmicroseism shows that if compared with Well X, the distribution offracture network in Well Y reservoir is poor and the oil and gasreservoir body is not effectively communicated. On the other hand, itcan be seen from FIG. 3 that the maximum daily gas output in Well Y is23.3×10⁴ m³, and the average daily gas output is 13.1×10⁴ m³ in thefirst year and 8.2×10⁴ m³ in the second year. Therefore, the fracturingconstruction scheme of shale reservoir, which is put forward throughparallel consideration of geological factors and engineering factors inthe present invention, is more reasonable. It can effectively optimizethe distribution of reservoir fracture network, increase the effectivecover area of fracture network, and stably improve the productioncapacity of single well for a long time. It has a certain guidingsignificance for the efficient and low-cost development of shalereservoirs. If compared with the prior art, the present invention hassignificant progress.

The above are not intended to limit the present invention in any form.Although the present invention has been disclosed as above withembodiments, it is not intended to limit the present invention. Thoseskilled in the field, within the scope of the technical solution of thepresent invention, can use the disclosed technical content to make a fewchanges or modify the equivalent embodiment with equivalent changes.Within the scope of the technical solution of the present invention, anysimple modification, equivalent change and modification made to theabove embodiments according to the technical essence of the presentinvention are still regarded as a part of the technical solution of thepresent invention.

What is claimed is:
 1. A method for evaluation of shale fracability under geology-engineering “double-track” system, comprising the following steps: S1: Divide a target horizontal fracturing interval into multiple sampling sections; S2: Establish a reservoir property evaluation factor of each sampling section, and calculate a geological evaluation index of the target horizontal fracturing interval according to the reservoir property evaluation factor of each sampling section; S3: Establish a brittleness factor, a natural fracture factor and a natural fracture opening factor of each sampling section, and then establish an engineering evaluation index of each sampling section according to the brittleness factor, the natural fracture factor and the natural fracture opening factor of each sampling section; The brittleness factor is calculated according to the following equations: $C_{i} = {{\lambda\left\lbrack {1 - {\exp\begin{pmatrix} M_{i} \\ {E_{i}} \end{pmatrix}}} \right\rbrack} + \begin{matrix} {\eta\left( {\sigma_{pi} - \sigma_{ci}} \right)} \\ \sigma_{pi} \end{matrix}}$ λ + η = 1 Where, C_(i) is the brittleness factor of the sampling section i based on a stress-strain curve, dimensionless; λ and η are standardized coefficients, dimensionless; M_(i) is a softening modulus of the sampling section i, in gigapascals (Gpa); E_(i) is an elasticity modulus of the sampling section i, in GPa; σ_(pi) is a peak strength obtained from a triaxial compression test at the sampling section i, in megapascals (Mpa); σ_(ci) is a participating strength obtained from the triaxial compression test at the sampling section i, in MPa; The natural fracture factor is calculated according to the following equations: $\underset{Fi}{P} = \frac{P_{fi} - P_{f\min}}{P_{f\max} - P_{f\min}}$ $P_{fi} = \frac{2E^{2}}{\left( {\underset{1i}{K^{2}} + \underset{2i}{K^{2}}} \right)\underset{i}{\upsilon}}$ $\underset{1i}{K} = {\underset{i}{0.3172\rho} + \frac{0.0457}{V_{ci}} + \underset{i}{0.2131{\ln({DT})} \times 0.5041}}$ $\underset{2i}{K} = {\underset{i}{2.1332\rho} + \frac{0.0768}{V_{ci}} + \underset{i}{1.1886{\ln({DT})} \times 9.1808}}$ Where, P_(Fi) is the natural fracture factor of the sampling section i, dimensionless; P_(fi) is a representation number of natural fracture development degree in sampling section i, ×10⁶ m⁻¹; P_(fmax) and P_(fmin) are respectively maximum and minimum representation numbers of natural fracture development degree in all sampling sections, ×10⁶ m⁻¹; K_(1i) and K_(2i) are respectively Type I and Type II fracture toughness of the sampling section i, in MPa·m^(0.5); ν_(i) is an average static Poisson's ratio of the sampling section i, dimensionless; ρ_(i) is an average shale density of the sampling section i, in g/cm³; V_(ci) is an average mud content of the sampling section i, in percentage (%); DT_(i) is an average acoustic time difference of the sampling section i, in μs/m; The natural fracture opening factor is calculated according to the following equations: $\begin{matrix} {\underset{Ti}{P} = \frac{P_{t\max} - P_{ti}}{P_{t\max} - P_{t\min}}} & (12) \end{matrix}$ $\underset{ti}{P} = {{\underset{xi}{\sigma}{\underset{1i}{l}}^{2}} + {\underset{yi}{\sigma}{\underset{2i}{l}}^{2}} + {\underset{zi}{\sigma}{\underset{3i}{l}}^{2}} + {2\tau\underset{{xyi}1i2i}{ll}} + {2\tau\underset{{yzi}2i3i}{ll}} + {2\tau\underset{{zxi}1i3i}{ll}}}$ $l_{1i} = \sqrt{\frac{1 - l_{3i}^{2}}{1 + {\tan^{2}\theta_{i}}}}$ l_(2i) = l_(1i)tan θ_(i) l_(3i) = cos α_(i) Where, P_(Ti) is the natural fracture opening factor of the sampling section i, dimensionless; P_(ti) is a fluid pressure when a natural fracture of the sampling section i is opened, in MPa; P_(tmax) and P_(tmin) are respectively maximum and minimum fluid pressures in all sampling sections to meet the opening of the natural fracture, in MPa; σ_(xi), σ_(yi) and σ_(zi) are respectively normal stress, tangential normal stress and vertical stress of a shaft in the sampling section i, in MPa; l_(1i), l_(2i) and l_(3i) are respectively cosine values of an included angle between the natural fracture and maximum horizontal principal stress, minimum horizontal principal stress and vertical stress in the sampling section i, dimensionless; τ_(xyi), τ_(yzi) and τ_(xzi) are respectively shear stress components of the sampling section i, in MPa; θ_(i) is the included angle between the natural fracture and the maximum horizontal principal stress, the minimum horizontal principal stress and the vertical stress in the sampling section i, in degrees (°); α₁ is a dip angle of the natural fracture in the sampling section i, in degrees (°); S4: Calculate an engineering evaluation index of the target horizontal fracturing interval according to the engineering evaluation index of each sampling section; S5: Evaluate the shale fracability of the target horizontal fracturing interval according to the geological evaluation index and the engineering evaluation index; S6: Establish a fracturing construction scheme for the target horizontal fracturing interval according to the geological evaluation index and the engineering evaluation index; wherein, in step S6, the fracturing construction scheme for the target horizontal fracturing interval is established according to the geological evaluation index and the engineering evaluation index, specifically as follows: when the geological evaluation index is within a range of [0,0.1] and the engineering evaluation index is within a range of [0,1.0], fracturing construction of the target horizontal fracturing interval will be abandoned when the geological evaluation index is within a range of [0.1,0.4] and the engineering evaluation index is within a range of [0,0.7], and when the geological evaluation index is within a range of [0.4,0.7] and the engineering evaluation index is within a range of [0.7,1.0], slickwater fracturing+fiber temporary plugging and diverting will be carried out for the target horizontal fracturing interval when the geological evaluation index is within the range of [0.1,0.4], and the engineering evaluation index is within the range of [0.7,1.0], slickwater fracturing will be carried out for the target horizontal fracturing interval; when the geological evaluation index is within the range of [0.4,0.7] and the engineering evaluation index is within a range of [0,0.3], and when the geological evaluation index is within a range of [0.7,1.0] and the engineering evaluation index is within a range of [0.3,0.7], large-scale fracturing+fiber temporary plugging and diverting+medium multi-clustered volume fracturing will be carried out for the target horizontal fracturing interval, the large-scale fracturing means that liquid intensity for fracturing is 150% of that for the slickwater fracturing when the geological evaluation index is within the range of [0.4,0.7] and the engineering evaluation index is within the range of [0.3,0.7], and when the geological evaluation index is within the range of [0.7,1.0] and the engineering evaluation index is within the range of [0.7,1.0], large-scale fracturing+fiber temporary plugging and diverting will be carried out for the target horizontal fracturing interval; when the geological evaluation index is within the range of [0.7,1.0] and the engineering evaluation index is within the range of [0,0.3], large-scale fracturing+fiber temporary plugging and diverting+multi-clustered volume fracturing will be carried out for the target horizontal fracturing interval.
 2. The method for evaluation of the shale fracability under the geology-engineering “double-track” system according to claim 1, wherein, in step S2, the reservoir property evaluation factor is calculated according to the following equations: e_(i) = a₁φ_(i)^(′) + b₁ω_(i)^(′) a₁ + b₁ = 1 $\varphi_{i}^{\prime} = \frac{\left( {\varphi_{ie} - \varphi_{\min}} \right)}{\left( {\varphi_{\max} - \varphi_{\min}} \right)}$ $\omega_{i}^{\prime} = \frac{\left( {\omega_{i} - \omega_{\min}} \right)}{\left( {\omega_{\max} - \omega_{\min}} \right)}$ Where, e_(i) is the reservoir property evaluation factor of the sampling section i, dimensionless; a₁ and b₁ are weight coefficients of physical properties, dimensionless; ϕ_(i)′ is a porosity of the sampling section i, dimensionless; ω_(i)′ is a total organic carbon content of the sampling section i, dimensionless; ϕ_(ie) is an effective porosity of the sampling section i, in %; ϕ_(max) and ϕ_(min) are respectively maximum effective porosity and minimum effective porosity in all sampling sections, in %; ω_(i) is a total organic carbon content of the sampling section i, in %; ω_(max) and ω_(min) are respectively maximum total organic carbon content and minimum total organic carbon content in all sampling sections, in %.
 3. The method for the evaluation of the shale fracability under the geology-engineering “double-track” system according to claim 2, wherein when the effective porosity of the sampling section is greater than or equal to 4.5%, a₁ and b₁ are 0.65 and 0.35 respectively; and when the effective porosity of the sampling section is less than 4.5%, a₁ and b₁ are 0.55 and 0.45 respectively.
 4. The method for the evaluation of the shale fracability under the geology-engineering “double-track” system according to claim 2, wherein, in step S2, the geological evaluation index is calculated according to the following equation: $A = \frac{\left( {\overset{\_}{e} - e_{\min}} \right)}{\left( {e_{\max} - e_{\min}} \right)}$ Where, A is the geological evaluation index of the target horizontal fracturing interval, dimensionless; ē is an average reservoir property evaluation factor for all sampling sections, dimensionless; e_(max) and e_(min) are respectively maximum and minimum reservoir property evaluation factors in all sampling sections, dimensionless.
 5. The method for the evaluation of the shale fracability under the geology-engineering “double-track” system according to claim 1, wherein, in step S3, the engineering evaluation index is calculated according to the following equation: $\frac{3}{f_{i}} = {\frac{1}{C_{i}} + \frac{1}{P_{Fi}} + \frac{1}{P_{Ti}}}$ Where, f_(i) is the engineering evaluation index of the sampling section i, dimensionless; C_(i) is the brittleness factor of the sampling section i based on the stress-strain curve, dimensionless; P_(Fi) is the natural fracture factor of the sampling section i, dimensionless; P_(Ti) is the natural fracture opening factor of the sampling section i, dimensionless.
 6. The method for the evaluation of the shale fracability under the geology-engineering “double-track” system according to claim 5, wherein, in step S4, the engineering evaluation index is calculated according to the following equation: $B = \frac{\left( {f - f_{\min}} \right)}{\left( {f_{\max} - f_{\min}} \right)}$ Where, B is the engineering evaluation index of the target horizontal fracturing interval, dimensionless; f is an average engineering evaluation factor for all sampling sections, dimensionless; f_(max) and f_(min) are maximum and minimum engineering evaluation factors for all sampling sections, dimensionless.
 7. The method for the evaluation of the shale fracability under the geology-engineering “double-track” system according to claim 1, wherein, in step S5, the shale fracability of the target horizontal fracturing interval is evaluated according to the geological evaluation index and the engineering evaluation index, specifically as follows: When the geological evaluation index is within the range of [0,0.1] and the engineering evaluation index is within the range of [0,1.0], the target horizontal fracturing interval is not fracturable; When the geological evaluation index is within a range of [0.1,1.0] and the engineering evaluation index is within the range of [0,1.0], the target horizontal fracturing interval is fracturable. 